This command performs variable substitutions, command substitutions,
and backslash substitutions on its string argument and
returns the fully-substituted result.
The substitutions are performed in exactly the same way as for
Tcl commands.
As a result, the string argument is actually substituted twice,
once by the Tcl parser in the usual fashion for Tcl commands, and
again by the subst command.

If any of the -nobackslashes, -nocommands, or
-novariables are specified, then the corresponding substitutions
are not performed.
For example, if -nocommands is specified, command substitution
is not performed: open and close brackets are treated as ordinary characters
with no special interpretation.

Note that the substitution of one kind can include substitution of
other kinds. For example, even when the -novariables option
is specified, command substitution is performed without restriction.
This means that any variable substitution necessary to complete the
command substitution will still take place. Likewise, any command
substitution necessary to complete a variable substitution will
take place, even when -nocommands is specified. See the
EXAMPLES below.

If an error occurs during substitution, then subst will return
that error. If a break exception occurs during command or variable
substitution, the result of the whole substitution will be the
string (as substituted) up to the start of the substitution that
raised the exception. If a continue exception occurs during the
evaluation of a command or variable substitution, an empty string
will be substituted for that entire command or variable substitution
(as long as it is well-formed Tcl.) If a return exception occurs,
or any other return code is returned during command or variable
substitution, then the returned value is substituted for that
substitution. See the EXAMPLES below. In this way, all exceptional
return codes are
“caught”
by subst. The subst command
itself will either return an error, or will complete successfully.

The continue and break exceptions allow command substitutions to
prevent substitution of the rest of the command substitution and the
rest of string respectively, giving script authors more options
when processing text using subst. For example, the script